This invention generally relates to a drive system for rotating an assembly about an axis. More particularly, this invention relates to drive system suitable for remotely adjusting an external vehicle mirror housing.
Automobiles and many other motor vehicles utilize one or more adjustable rearview mirrors to allow the operator to view conditions to the sides and rear of the vehicle. These rearview mirrors may be mounted within the vehicle cabin or outside, on the frame of the vehicle. Proper orientation of the rearview mirrors is important to safe operation of the vehicle, but it can be difficult to adjust an external mirror, especially during operation of the vehicle. Accordingly, it is known to provide a drive system for remotely adjusting an external mirror from the vehicle cabin. One example of such a drive system is described in U.S. Pat. No. 4,787,726 to Hendricks, the disclosure of which is hereby incorporated herein by reference.
FIG. 1 illustrates a drive system according to one known design. The drive system, generally identified in FIG. 1 as element 10, is contained within a drive housing 12 that is receivable in or adjacent to a mirror housing (not shown). The mirror housing is rotatably associated with a vertical rod 14, which is substantially stationary with respect to the vehicle frame (also not shown). The vertical rod 14 extends through the drive housing 12 and defines an axis about which the drive housing 12 is rotatable. The vertical rod 14 is encircled by an axis gear 16 inside of the drive housing 12. Typically, the axis gear 16 is a worm gear or worm wheel which is normally stationary with respect to the vertical rod 14. Under certain conditions, described below, the axis gear 16 may be forced to rotate about the vertical rod 14 in order to rotate the drive housing 12 and the mirror housing.
The axis gear 16 engages a drive gear 18, which is mounted on a shaft 20 for rotation therewith. The drive gear 18 is typically a worm, but may take a different form depending on the geometry of the axis gear 16. The shaft 20 is supported by bearings 22 adjacent to a front wall 24 and a rear wall 26 of the drive housing 12. A worm gear 28 is also mounted on the shaft 20 for rotation therewith. The worm gear 28 engages a worm 30 mounted on a shaft 32 for rotation therewith. A spur gear 34 is also mounted on the shaft 32 for rotation therewith. The spur gear 34 engages a pinion 36 mounted on a shaft 38, which is rotated by an electric motor 40.
In use, the pinion 36 is rotated by the motor 34, which ultimately rotates the drive gear 18. The torque supplied by the drive gear 18 is not sufficient to rotate the axis gear 16, so the drive gear 18 travels about the axis gear 16. This movement of the drive gear 18 causes the associated drive housing 12 to rotate about the axis defined by the vertical rod 14. Since the mirror housing is fixed to the drive housing 12, it is effectively adjusted by the movement of the drive gear 18.
As mentioned above, the axis gear 16 is normally rotationally connected to the vertical rod 14 via a slip clutch mechanism. Normally gear 16 will not rotate with respect to shaft 14 unless it is acted upon by a sufficient torque. The torque applied by operation of the drive system 10 is normally not sufficient to rotate the axis gear 16 with respect to shaft 14. However, there may be instances where either an external force is sufficient to force rotation or the drive system may force the axis gear 16 to rotate with respect to shaft 14 if the housing is being prevented from rotating about the axis (when the motor is running) by an obstruction or abnormal friction. For example, the torque applied by an operator manually adjusting the mirror housing or by an object striking the mirror housing will cause the axis gear 16, and hence the drive housing 12 and mirror housing, to rotate about the vertical rod 14. The clutch will then slip, preventing damage to the gears and to the motor itself. Of course, this is an optional feature of the drive system 10, but it is useful in preventing damage to the gears. A number of forced rotation systems are known, including one described in Hendricks and another described in U.S. Pat. No. 6,022,113 to Stolpe et al., the disclosure of which is hereby incorporated herein by reference.
It is desirable that there be no uncertainty in the rotational position of housing 12 (and thus the mirror glass) with respect to shaft 14 (and thus the vehicle frame). Such uncertainty or “play” means that the mirror glass does not stay in a fixed position with respect to the vehicle when subjected to outside vibration/excitation forces. Rotational position uncertainty has multiple sources depending on the mechanism, and these are typically additive i.e. total uncertainty equals the sum of the contributing factor uncertainties. Backlash in the mesh of the gears is one contributing factor to this uncertainty.
The present invention doesn't affect backlash, as it would strictly be defined. Backlash is a characteristic of gear tooth mesh wherein only one side of a given tooth can be in contact with the mating gear at a given position or point in time. There is space on the other side of that tooth and therefore the relative positions of the two gears in mesh have some uncertainty. With this uncertainty of relative position, there can be uncontrolled instantaneous rotational acceleration.
Having the final drive (18) be a worm has the advantage that the mechanism is not subject to back drive (i.e. the gears will not be driven when torque is applied on the housing 12 with respect to shaft 14 and axis gear 16) if the lead angle of the worm is low enough. However worms have the characteristic that transmission of torque is accompanied by thrust loads in the direction of the worm's axis.
If the worm is not sufficiently constrained in its axial direction, external forces on the mirror will be able to effect rotation of housing 12 about shaft 14. This can occur even if there is no backlash in the mesh of drive gear 18 to axis gear 16 (i.e. a given tooth on axis gear 16 has contact on both sides with worm 18 a.k.a. double flank contact). This is because the worm 18 will slide along its axis until it contacts the bearings 22 which contact housing 12 at either rear wall 26 or front wall 24, depending on direction.
Therefore it is desirable to have low axial play in the assembly containing worm 18. As shown in FIG. 1 (prior art), the amount of axial play in the 18 worm assembly is determined by the tolerances on the distance between 42-42 the ribs 42, the thickness of bearings 22, and the lengths of the gears 18 & and 28. A tolerance of +/−0.005 on each of these five features produces overall tolerance of 0.050″. To accommodate these tolerances and not have interference a nominal clearance of 0.025″ would have to be provided, which could then result in 0 to 0.050″ clearance depending on the sizes of the five features in the population of parts. At 16-18a mesh distance of 0.83 in gears 16 and 18, 0.050″ axial play in worm 18 allows nearly 3.5° in rotation of housing 12 (and hence the mirror glass) when the mirror is subjected to road and vehicle induced vibration/excitation. It is very difficult for the driver to use or view the image from a mirror with such movement. Therefore the tolerances employed in a mechanism of this design must be much tighter, and manufacturing expenses will be higher. Even with tighter tolerances, zero axial play can not be feasibly achieved with this approach. This is a drawback of this approach.
Another possible drawback of known drive systems, especially when the drive gear 18 is a worm gear or a helical gear, is that a great deal of thrust may be transmitted through the drive gear 18 and the shaft 20 to the front and rear walls 24 and 26 of the drive housing 12. Accordingly, the front and rear walls 24 and 26 must be reinforced to prevent deformation. For example, FIG. 1 shows reinforcing ribs 42 used to prevent the front and rear walls 24 and 26 from bowing outwardly under the force transmitted through the shaft 20.
Accordingly, a general object and aspect of the present invention is to provide an improved drive system which overcomes the above-described drawbacks of known systems.
Other aspects, objects and advantages of the present invention, including the various features used in various combinations, will be understood from the following description according to preferred embodiments of the present invention, taken in conjunction with the drawings in which certain specific features are shown.